Some of widely known electrophotographic light-sensitive elements include a light-sensitive layer primarily comprising selenium, zinc oxide, cadmium sulfide or other inorganic photoconductive compound. However, they are not satisfactory in thermal stability, operating life and other various characteristics. Beside this, they have one or other toxicity which requires a particular attention in manufacturing them.
In contrast, electrophotographic light-sensitive elements having a light-sensitive layer primarily comprising an organic photoconductive compound have many advantages such as easier manufacturing process, less cost, easy handling and excellent thermal stability in most cases as compared to selenium light-sensitive elements, and they arouse a great interest. The most widely known organic photoconductive compound of this sort is poly-N-vinylcarbazole. An electrophotographic light-sensitive element is already used in practice that has a light-sensitive layer primarily comprising a charge-transfer complex made from the foregoing poly-N-vinylcarbazole and 2,4,7-trinitro-9-fluorenone or other Lewis acid.
Some known electrophotographic light-sensitive elements have a separate operation type light-sensitive layer of a multi-layer type or a separate type in which the carrier generating function and the carrier transport function of a photoconductive element are shared by different agents. For example, an electrophotoconductive light-sensitive element is already used in practice that comprises a carrier generating layer in the form of, for example, an amorphous selenium film and a carrier transport layer primarily including poly-N-vinylcarbazole.
Further, some efforts are done to make both a carrier generating agent and a carrier transport agent from organic photoconductive compounds to form an electrophotographic light-sensitive element including the foregoing separate operation type light-sensitive layer. In this case, typical carrier generating agents are azo coloring matter, phthalocyanine coloring matter, anthraquinone coloring matter, perylene coloring matter, cyanine coloring matter, thiapyrylium coloring matter, squaric acid derivative dye, etc. Typical carrier transport agents are amine derivative, oxazole derivative, oxadiazole derivative, triphenylmethane derivative, etc.
When selected carrier generating agent and carrier transport agent do not have a film-forming capacity by themselves, they each are used together with any suitable binding agent to form a film. In these circumstances, some known electrophotographic light-sensitive elements have a layer including a carrier generating agent and a layer including a carrier transport agent which both layers are in the form of multiple layers on a conductive support base. Other known electrophotographic light-sensitive elements have a layer in which a carrier generating agent and a carrier transport agent are dispersed.
However, since these electrophotographic light-sensitive elements using an organic photoconductive compound as a carrier generating agent still have a low sensitivity of the photoconductivity and a short operating life as compared to those using an inorganic photoconductive compound, there are few elements that are acceptable practically.